Magnetic resonance spectroscopic imaging (MRSI) is often used to investigate metabolite concentration changes. Specifically, MSRI can be used to non-invasively investigate metabolite concentration changes in the brain correlated to neurological and psychiatric diseases, brain tumors, and radiation damage. MRSI has been performed at field strengths such as 1.5 T and 3 T. However, with transition to higher field strengths come challenges, such as greater radiofrequency power deposition, which may approach specific absorption rate (SAR) safety limits, transmitted radiofrequency (B1) inhomogeneity, and more severe spatial shifts in the excited volume for metabolites resonating at two different frequencies and/or chemical shift localization (CSL) errors. The 180 degree radiofrequency pulses in the conventional MRSI sequences are particularly susceptible to the variation in transmitted radiofrequency field and severe chemical shift at magnetic field strengths of 7 T. This results in signal attenuation in multiple regions of the excited volume.
Spectroscopic imaging is most commonly performed using a point resolved spectroscopy (PRESS) sequence which uses a 90 degree radiofrequency pulse followed by at least two 180 degree radiofrequency pulses. The radiofrequency pulses are selected along different spatial dimensions and form a double spin echo over the volume of interest. In MRSI, further spatial localization is performed within the volume of interest through the use of phase encodes or oscillating read out gradients. The 180 degree radiofrequency pulses used in the PRESS sequence are particularly sensitive to the substantial variation in B1 that exists at higher field strengths, resulting in drastic signal attenuation in multiple regions of the brain. These pulses are also very susceptible to CSL error, which scales linearly with field strength, resulting in substantial shifts in the selected volume for metabolites that are sparsely separated in frequency. Although high bandwidth radiofrequency pulses may mitigate the CSL error, they come at the cost of increased SAR.
Excess power deposition during the scan is also a challenge at higher field strengths. Radiofrequency power deposition, as measured by SAR, increases quadratically with field strength and 180 degree RF pulses are particularly SAR intensive. At higher field strengths, radiofrequency pulse sequences containing two or more 180 degree pulses, such as the double spin echo PRESS sequence, may reach or exceed SAR safety limits leading to an imaging delay or termination of the acquisition.
In order to address some of these challenges, several variants of the conventional PRESS sequence have been proposed. Adiabatic pulses create B1-insensitive refocusing and reduced CSL error. A 3D adiabatic MRSI sequence, such as Localized Adiabatic Selective Refocusing (LASER), achieves volume selection using adiabatic full passage refocusing pulses along all three spatial axes. However, adiabatic pulses are SAR-intensive and typically deposit quadratic phase across the spatially selective dimension which must be refocused, most often with a second, identical, adiabatic full passage (AFP) pulse across the same axis. A fully 3D MRSI sequence, such as LASER, requires at least 6 matched adiabatic refocusing pulses, resulting in an extended minimum echo time, and very high SAR.
The semi-LASER sequence uses a non-adiabatic selective excitation, combined with two pairs of adiabatic refocusing pulses. This enables a shorter echo time and results in lower SAR when compared to LASER. However, the 4 high-SAR adiabatic refocusing pulses still limit the application of semi-LASER MRSI in vivo, by extending total scan time.
Adiabatic spectral spatial pulses (SPSP) have been used in MRSI sequences to simultaneously provide B1-insensitive selection and reduced CSL error. Since the spatial selectivity is achieved by linear-phase spatial sub-pulses, pairs of pulses are not required to refocus quadratic phase in the spatial dimensions. Quadratic phase is deposited in the spectral dimension by the first adiabatic 180 degree pulse, but this quadratic phase is refocused by a second identical SPSP adiabatic 180 degree pulse. By obviating the need for pairs of adiabatic refocusing pulses for each spatial dimension, the use of adiabatic SPSP pulses to select the volume of interest may be used to reduce total SAR when compared to semi-LASER.
Hyperbolic secant adiabatic pulses have been used as spectral envelopes to create the adiabatic SPSP pulses. This approach, however, resulted in a spectral bandwidth limited by peak radiofrequency, necessitating a spectrally interleaved approach to cover the full range of interesting brain metabolites.